Nitrosamine Formation in Amine Scrubbing at Desorber Temperatures

Jun 23, 2014 - Amine scrubbing is a thermodynamically efficient and industrially proven method for carbon capture, but amine solvents can nitrosate in...
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Nitrosamine Formation in Amine Scrubbing at Desorber Temperatures Nathan A. Fine, Mark J. Goldman, and Gary T. Rochelle* The University of Texas at Austin, McKetta Department of Chemical Engineering, 200 E Dean Keeton Street Stop C0400, Austin, Texas 78712-1589, United States S Supporting Information *

ABSTRACT: Amine scrubbing is a thermodynamically efficient and industrially proven method for carbon capture, but amine solvents can nitrosate in the desorber, forming potentially carcinogenic nitrosamines. The kinetics of reactions involving nitrite and monoethanolamine (MEA), diethanolamine (DEA), methylethanolamine (MMEA), and methyldiethanolamine (MDEA) were determined under desorber conditions. The nitrosations of MEA, DEA, and MMEA are first order in nitrite, carbamate species, and hydronium ion. Nitrosation of MDEA, a tertiary amine, is not catalyzed by the addition of CO2 since it cannot form a stable carbamate. Concentrated and CO2 loaded MEA was blended with low concentrations of N-(2hydroxyethyl) glycine (HeGly), hydroxyethyl-ethylenediamine (HEEDA), and DEA, secondary amines common in MEA degradation. Nitrosamine yield was proportional to the concentration of secondary amine and was a function of CO2 loading and temperature. Blends of tertiary amines with piperazine (PZ) showed n-nitrosopiperazine (MNPZ) yields close to unity, validating the slow nitrosation rates hypothesized for tertiary amines. These results provide a useful tool for estimating nitrosamine accumulation over a range of amine solvents.

1. INTRODUCTION Existing coal- and gas-fired power plants represent the largest point sources for CO2 emissions worldwide. These emissions are the major driving force behind global warming, which could increase global temperatures by 6 °C by 2050. Carbon capture and sequestration will be a necessary tool to keep temperatures below the target of a 2 °C rise,1 and amine scrubbing can fulfill that role as a retrofit technology for both coal- and gas-fired power plants. However, amine scrubbing involves the direct contact of the solvent with the contaminated flue gas, allowing the formation of hazardous byproducts such as nitrosamines. Nitrosamines can escape from the CO2 capture system either through accidental spills, reclaimer waste, or gaseous emissions,2,3 so it is important to understand and limit their formation in the CO2 capture system itself. Nitrosamines primarily form when NO2 from the flue gas reacts with the amine solvent. NO2 equilibrates with NO in the flue gas to form N2O3, which can directly react with amines to form nitrosamines in the absorber.4−6 NO2 by itself can also absorb into solution as nitrite without forming nitrosamines. At the ppm of NOx levels found in flue gas, NOx equilibrium shifts away from N2O3 in favor of NO and NO2. This leads to selective NO2 absorption as nitrite with less than 10% of the total absorbed NOx directly nitrosating the amine.7,8 Nitrite from NO2 absorption then travels to the high-temperature desorber where it can nitrosate the amine.9 Since nitrosamine formation is dominated by high-temperature nitrosation, the kinetics and yields for nitrosamine formation under desorber conditions determine the accumulation rate of nitrosamines in the scrubber. © 2014 American Chemical Society

Nitrosation from nitrite has been extensively studied at conditions simulating gastric juices. In this low temperature acidic environment, nitrosation is second order in nitrite, first order in free amine concentration, and dependent on pH because the nitrosation mechanism goes through nitrous acid.10−12 There is no nitrous acid present under the basic conditions in an amine scrubber, but nitrite can still rapidly nitrosate amines in the presence of a formaldehyde catalyst.13,14 Computational modeling has shown that carbamate groups might also catalyze nitrosation formation at high pH but would inhibit formation at low pH.15,16 High temperature piperazine (PZ) nitrosation is indeed catalyzed by carbamate with nitrosation rates following a first order dependence on total carbamate concentration.9 This research focuses on characterizing high temperature nitrosation for several other secondary amines as well as primary, tertiary, and hindered amines that are being considered for amine scrubbing. Furthermore, this research explores nitrosamine yield in a variety of amine blends under desorber conditions in order to determine expected accumulation levels of nitrosamines in amine scrubbers.

2. EXPERIMENTAL METHODS 2.1. Amine Preparation. A list of all chemicals used in this research can be found in Supporting Information A. N-(2Received: Revised: Accepted: Published: 8777

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Table 1. Nitrosation Observed and Calculated Kinetics experiment

α (mol CO2/mol N)

kobs (1/s × 10−6)

T (°C)

amine (M)

120 100 135 120 135 120 135 120 150

5.0 4.9 5.1 5.1 2.6 2.6 2.5 2.5 2.5

0.35 0.35 0.18 0.18 0.36 0.36 0.17 0.17 0.17

7.67 8.06 7.89 8.17 7.39 7.67 7.89 8.17 7.39

79 ± 4 9.7 55 ± 3 11.3 227 ± 5 44.0 35.0 8.4 97 ± 4

100 80 100 80 100 80 100 80 120 60

4.1 4.1 4.1 4.1 2.3 2.3 2.3 2.3 2.3 4.1

0.42 0.42 0.18 0.18 0.42 0.42 0.18 0.18 0.18 0.42

7.66 8.21 7.99 8.54 7.73 8.27 7.98 8.52 7.58 8.60

545 ± 15 81 ± 4 77.6 10.7 217 ± 5 35.2 46.8 6.8 264 ± 11 9.4

80 100 80 100 100 120

3.7 3.7 0.9 0.9 0.9 0.9

0.19 0.19 0.38 0.38 0.19 0.19

9.27 8.92 8.85 8.50 8.81 8.49

2.5 15.0 2.8 15.5 4.5 25.0

120 120

1.4 1.4

0.0003 0.16

7.23 7.23

pHexp

kcalc (1/s × 10−6)

MEA 1 2 3 4 5 6 7 8 9 DEA 10 12 13 14 15 16 17 18 19 20 MMEA 21 22 23 24 25 26 MDEA 27 28

(1)

ln[NO2−] = ln[NO2i−] − kobst

(2)

± 0.3 ± 0.3 ± 0.7 ± 0.3

± 0.2 ± 0.3 ± 0.4 ± 0.4 ± 0.4 ± 1.1 ± ± ± ± ± ±

87 10.2 61 13.9 196 44.6 29.4 6.7 119.0 464 62 90.9 12.2 219 29.3 53.7 7.2 263 10.7

0.1 0.5 0.1 0.6 0.3 0.9

69 ± 4.1 190 ± 7.4

2.1 14.3 2.7 18.5 4.5 25.8 N/A N/A

extrapolation of the third-order rate constant outside of experimental conditions (eq 3). The pH of the solution under nitrosation conditions was estimated by first measuring the pH of solution at room temperature. The pH under experimental conditions was then estimated by adjusting for the pKa temperature dependence (eq 4). 20,21 The pKa at experimental conditions was extrapolated using the enthalpy of dissociation of the amine whenever high temperature pKa data were lacking (eq 5).

Hydroxyethyl)glycine (HeGly) was the only amine not commercially available, so it was synthesized by adding 0.5 M sodium chloroacetate into 5 M monoethanolamine (MEA) and reacting at 65 °C for 6 h to yield a stock solution of 0.5 M HeGly in 4.5 M MEA.17 Amine solutions were gravimetrically loaded with CO2 using a sparger. The final amine concentration and loading (α) defined as moles CO2/mol N were measured using acid titration and total inorganic carbon analysis (TIC), respectively.18 2.2. Nitrosation Kinetics. The prepared amine samples were spiked with approximately 50 mM of sodium nitrite (NaNO2) and pipetted into thermal cylinders. The cylinders were heated in convection ovens or water baths at a set temperature ranging from 60−150 °C and then removed periodically to form a time series. Nitrosation kinetics were measured by the disappearance of nitrite in solution using anion chromatography.19 Nitrosation under desorber conditions is first order in nitrite,9 so the time series were regressed as pseudo-first-order decompositions of nitrite using eqs 1 and 2. d[NO2−] = −kobs[NO2−] dt

± 0.2

kcalc = k 3e Ea / R[(1/373K) − (1/ Texp)]10−pHexp[CO2 absorbed] (3)

pHexp = pHRT − (pK aRT − pK aexp)

(4)

pK aexp = pK aRT − log(e−ΔHDiss/ R[(1/ Texp) − (1/ TRT)])

(5)

2.3. Nitrosamine Yield in MEA. Concentrated, loaded solutions of MEA were spiked with 50 mM NaNO2. The approximately 50 g solution was separated into individual beakers, and each beaker was spiked with 0−100 mM of additional secondary amine. After thorough mixing, the final solutions were pipetted into the thermal cylinders and heated. Cylinders were removed from the oven as a single set so that each cylinder was heated under identical conditions but had differing concentrations of secondary amine. The samples were analyzed for remaining nitrite as well as the nitrosamine from the added secondary amine. The nitrosamine yield is defined as the fraction of nitrite that nitrosated the secondary amine (eq

The observed rate constants were then fit as a function of temperature, pH, and total absorbed CO2; temperature dependence was centered at 373 K in order to avoid 8778

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6). The experimental run time was chosen to maximize nitrosation while keeping secondary nitrosamine decomposition insignificant (approximately 16 h at 120 °C and 3 h at 150 °C). [NNO] yield = − [NO2i ] − [NO2f −]

Table 2. Nitrosation Parameters for Primary and Secondary Amines

(6)

2.4. Nitrosamine Yield in PZ-Promoted Tertiary and Hindered Amines. Frequently, PZ is used as an absorption rate promoter in tertiary and hindered amine solvents. To study high temperature nitrosamine yield in these solvents, blends with 2 mol of PZ for every seven moles of tertiary or hindered amine were loaded to α = 0.25 and spiked with 50 mM NaNO2. The samples were heated at 150 °C for 3 h and then analyzed for both MNPZ and nitrite; yield was again determined using eq 6. The MNPZ formed during the experiment can further nitrosate to dinitrosopiperazine (DNPZ). However, since MNPZ concentration is much lower than PZ, there is negligible DNPZ formation during these experiments. 2.5. Nitrosamine Analysis. Nitrosamines were analyzed using a combination of total nitrosamine analysis and high pressure liquid chromatography (HPLC). Standards were readily available for n-nitrosopiperazine (MNPZ) and nitrosodiethanolamine (NDELA), so these were directly analyzed on the HPLC as previously described.9,22 The method uses UVdetection at 240 nm, the eluents are 10 mM ammonium carbonate (NH4)2CO3 (pH = 9.1) polar phase and acetonitrile (ACN) nonpolar phase, and the column is a Dionex Polar Advantage II, 4 × 250 mm. Nitroso-n-2-(hydroxyethyl)glycine (NHeGly) and nitrosohydroxyethyl-ethylenediamine (NHEEDA) standards were not available, so the standards were synthesized by adding 50 mM NaNO2 to the CO2 loaded parent amine. The standards were then analyzed using the total nitrosamine method.23−25 Briefly, the nitrosamine standards were reacted with hydrobromic acid in a mixture of ethyl acetate, acetic acid, and acetic anhydride at ambient conditions, forming 1 mol of nitric oxide (NO) gas for every mole of nitrosamine. The NO gas was analyzed using a chemiluminescent NOx analyzer to give nitrosamine concentration.26 The results from the total nitrosamine analysis were used to calibrate the HPLC peak areas for NHeGly and NHEEDA (Supporting Information B). 2.6. Nitrosamine Safety. While the toxicity of nitrosamines varies widely, all nitrosamines were treated as if they were very toxic and were handled with extreme caution during all experiments. All samples containing nitrosamines were clearly marked and stored in vented hoods. All experiments used less than 50 mM of nitrosamine and analysis was done with less than 1 mM of nitrosamine in the diluted sample.

amine

k3 (1/M2·s)

Ea (kJ/mol)

MEA DEA MMEA PZ9

680 12000 17000 8500

73 42 62 84

Figure 1. Pseudo-first-order nitrite decomposition (experiment 5, 2.6 M MEA, α = 0.36, T = 135 °C).

suggesting a very good fit for first order nitrite decay. Nitrite decay is also hypothesized to be first-order in hydronium ion activity and carbamate concentration as shown in PZ.9 Thus, the observed rate constants were fit to eq 3 with k3 and Ea as free parameters. The calculated rate constant was usually within 10% of the observed rate constant even though the rates vary by over 2 orders of magnitude (Figure 2). All primary and secondary amines tested are therefore carbamate catalyzed and pH dependent, corroborating the mechanism previously proposed for PZ nitrosation.9 MDEA is a tertiary amine, so it does not form the carbamate necessary for nitrosation catalysis. Loaded MDEA nitrosated only 3 times faster than unloaded MDEA, even though bicarbonate concentration increased by a factor of 500 (Figure 3). Since the rate increase was not proportional to the increase in CO2 loading, MDEA nitrosation does not follow the same

3. RESULTS AND DISCUSSION 3.1. Nitrosation Kinetics. Table 1 summarizes all of the observed pseudo-first-order nitrosation rates for MEA, DEA, methylethanolamine (MMEA), and methyldiethanolamine (MDEA). The observed rate was computed from eq 2 along with the standard error. The calculated rate is given by eq 3 with values for the third order rate constant (k3) and activation energy (Ea) in Table 2. Several nitrosation experiments were allowed to go to completion with nitrite decomposition following first-order decay up to the nitrite detection limit (Figure 1). The standard error from fitting the data to eq 2 was always less than 5%,

Figure 2. Observed rate compared with third-order nitrosation model for MEA (diamonds), DEA (squares), and MMEA (triangles) (experiments 1−26). 8779

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[MEACOO−] + [2°AmCOO−] = [MEAH+] + [2°AmH+] (12) −

−

MEA + 2°AmCOO ←⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MEACOO + 2°Am K cMEA / Kc2 ° Am

(13a) −

KcMEA [MEACOO ][2°Am] = Kc2 ° Am [MEA][2°AmCOO−] MEA + 2°AmH+ ←⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MEAH+ + 2°Am K aMEA / K a2 ° Am

(13b) (14a)

+

K aMEA [MEAH ][2°Am] = K a2 ° Am [MEA][2°AmH+] Figure 3. Relatively small change in observed nitrosation rate of MDEA (blue) compared to change in bicarbonate concentration (red) (experiments 27 and 28).

Assuming that the secondary amine concentration is much less than MEA concentration and there is minimal bicarbonate formation, MEA speciation is given by eqs 15 and 16. Secondary amine speciation can then be solved with eqs 11, 13b, and 14b using Excel solver. To minimize speciation error, all experiments were done with α less than 0.4 so that bicarbonate accounts for less than 5% of total absorbed CO2.

mechanism as primary and secondary amine nitrosation. Instead, the rate increase could be caused by differences in pH at experimental conditions or the higher ionic strength in the loaded MDEA. 3.2. Nitrosamine Yield in MEA. MEA is the industry standard and most researched solvent for amine scrubbing. As a primary amine, MEA nitrosates into an unstable nitrosamine that immediately deaminates into a carbocation and nitrogen gas.27 Any nitrosamine accumulation in an MEA solvent will therefore not come from the MEA, but from secondary amines formed during MEA degradation. DEA, HeGly, and HEEDA are all secondary amine products stemming from MEA degradation.28,29 Furthermore, NDELA and NHeGly have been quantified in degraded MEA pilot plant samples.30 To study the formation of these nitrosamines in a synthetic degraded MEA sample, low concentrations of DEA, HeGly, and HEEDA were added to concentrated MEA and heated to desorber conditions. Assuming the nitrosation of secondary amines happens in parallel with the nitrosation of MEA, the nitrosamine yield can be calculated by eq 7. At low concentrations of secondary amine almost all of the nitrite reacts with MEA so that kobs2°Am ≪ kobsMEA. Equation 7 then simplifies to eq 8, which can be expanded by substituting the calculated rate for the observed rate constants (eq 9). The final form has no explicit pH dependence since both the primary and secondary amine will nitrosate at the same pH. yield =

kobs2 ° Am kobs2 ° Am + kobsMEA

(7)

yield ≈

kobs2 ° Am kobsMEA

(8)

yield ≈

(14b)

[MEACOO−] = [MEAH+] ≈ α[MEA T]

(15)

[MEA] ≈ (1 − 2α)[MEA T]

(16)

By substituting eq 13b into eq 9, it is apparent that nitrosamine yield is proportional to the relative concentrations of amines in solution (eq 17) where the reactivity (r) takes into account the speciation of the amines as well as relative nitrosation rates. yield = r

[2°Am] [MEA]

(17)

Experimentally, nitrosamine yield was proportional to the relative concentrations of amines with the linear regression of the reactivity parameter consistently giving standard errors less than 10% (Figure 4). There was consistently a small yield of

k 32 ° Ame Ea2°Am / R[(1/373K) − (1/ Texp)][2°AmCOO−]

Figure 4. Linear dependence of NDELA yield on [DEA]/[MEA] (experiment 36, α = 0.38, T = 150 °C).

k 3MEA e EaMEA / R[(1/373K) − (1/ Texp)][MEACOO−] (9)

NDELA in MEA solution without any added DEA. The NDELA most likely comes from small DEA impurities in the laboratory grade MEA as well as the formation of DEA in situ from MEA nitrosation.27,31 The reactivity is a function of loading since the speciation of CO2 between the amines changes with loading. A speciation model for DEA in MEA was built using carbamate stability

The amine carbamate concentrations can be found by solving the mass (eqs 10 and 11), charge (eq 12), and equilibria (eqs 13 and 14) balances for the CO2 speciation in solution. [MEA] + [MEACOO−] + [MEAH+] = [MEA T]

(10)

[2°Am] + [2°AmCOO−] + [2°AmH+] = [2°AmT]

(11) 8780

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3.3. Nitrosamine Yield in PZ-Promoted Tertiary and Hindered Amines. Tertiary and moderately hindered amines do not form carbamates; instead CO2 absorbs into solution as bicarbonate.33 Since an amine group is not taken up by the formation of carbamate, tertiary amines normally have higher capacities than primary or secondary amines. Unfortunately, tertiary amines also do not absorb CO2 quickly, so PZ is often added as an absorption rate promoter. Table 4 gives

constants and pKa data measured at lower temperatures and then extrapolating to reaction conditions.20,32 Calculated reactivity of DEA at 120 °C using eq 9 and kinetics from Table 2 overpredicts the experimental reactivity by approximately 30% for MEA loadings from α = 0.1 to 0.4 (Figure 5).

Table 4. MNPZ Yield in PZ-Promoted Tertiary Amine Blends (7 m Tertiary Amine, 2 m PZ, α = 0.25, T = 150 °C)

This error is to be expected given the uncertainties in estimating high temperature carbamate and protonate stability. DEA, HEEDA, and HeGly all have reactivities greater than one, so they nitrosate faster than MEA on a per mole basis (Table 3). If only 1% of the total amine in a degraded MEA solvent is present as these secondary amines, then 2−5% of absorbed NO2 can be expected to yield a secondary nitrosamine. Table 3 summarizes the results for nitrosamine formation in MEA spiked with low concentrations of secondary amines. The reactivity (r) represents the selectivity of secondary amine nitrosation over MEA as determined by the yield of secondary nitrosamine in the spiked MEA solution at varying concentrations of secondary amine.

29 30 31 32 33 34 35 36 HEEDA 37 38 39 40 HeGly 41 42 43 44

α (mol CO2/mol MEA)

120 120 120 120 120 120 150 150

DEA 0.10 0.16 0.20 0.30 0.38 0.40 0.16 0.38

120 120 150 150 120 120 150 150

rexp

rcalc

9.1 7.7 5.0 3.6 3.2 3.4 4.9 2.4

± ± ± ± ± ± ± ±

2.0 0.5 0.2 0.6 0.1 0.4 0.1 0.1

10.7 8.3 7.2 5.4 4.5 4.4

0.17 0.38 0.17 0.38

2.6 2.8 2.0 2.2

± ± ± ±

0.02 0.1 0.1 0.03

0.17 0.38 0.17 0.38

6.2 4.6 5.4 4.2

± ± ± ±

0.2 0.1 0.1 0.2

MNPZ yield

N/A 45 46 47 48 49

0.1−5 M PZ PZ/methyldiethanolamine (MDEA) PZ/triethanolamine (TEA) PZ/dimethylaminopropanol (DMAP) PZ/diethylaminoethanol (DEAE) PZ/aminomethylpropanol (AMP)

95 ± 11 85 102 93 96 79

4. ENVIRONMENTAL IMPACT 4.1. Effect of Nitrosamine Yield on Nitrosamine Accumulation. Secondary nitrosamines are thermally unstable and will decompose under the high temperatures of the desorber.26 Therefore, nitrosamines will accumulate in the carbon capture system to a steady state nitrosamine concentration where the rate of nitrosamine decomposition equals the rate of nitrosamine formation from NO2 absorption. This steady state nitrosamine concentration is dependent on the concentration of NO2 absorbed from the flue gas into the solvent, the nitrosamine yield from NO2 absorption, and the nitrosamine decomposition rate (eq 18). The steady state nitrosamine concentration can be estimated using nitrosamine decomposition rates at typical solventspecific desorber temperatures34 and the nitrosamine yield (Table 5). For undegraded MEA with only 0.1% of the amine

Table 3. Relative Nitrosation Reactivity of Secondary Amines Compared to MEA (Secondary Amine Varied 0−100 mM in 4−5 M MEA) T (°C)

amine blend

nitrosamine yield results for PZ-promoted tertiary amines. While these amines can nitrosate, they are not catalyzed by absorbed CO2 and thus do not compete well with PZ nitrosation. Even though PZ only makes up 22% of the total amine, it accounts for nearly all of the nitrosation.

Figure 5. Relative rate of nitrosation of DEA to MEA at 120 °C (experiments 29−34).

experiment

experiment

Table 5. Nitrosamine Accumulation in MEA, PZ, and 7 m MDEA/2 m PZ (NO2Abs = 5 × 1O−6 mol NO2/mol flue gas; G/L = 0.1 mol flue gas/L solvent) amine MEA, 0.1% as HeGly MEA, 1% as HeGly PZ 7 m MDEA/ 2 m PZ

desorber T (°C)

δ (mol NNO/mol NOxAbs)

kDes (1/s × 10−6)

NNO (mM)

120

0.0043

1.1

0.4

120

0.043

1.1

4.1

150 135

1 1

26.9 3.2

3.9 33.0

as HeGly, nitrosamine buildup is low. However, if MEA is degraded so that 1% of the total amine is HeGly, then the nitrosamine can build up to 4 mM. Nitrosamine accumulation in PZ will be limited to 4 mM even though 100% of the absorbed NO2 goes to forming MNPZ. This is due to the high desorber temperature used for PZ solvents, which readily decomposes MNPZ. Blends of tertiary amines with PZ will 8781

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(8) Fostås, B.; Gangstad, A.; Nenseter, B.; Pedersen, S.; Sjøvoll, M.; Sørensen, A. L. Effects of NOx in the flue gas degradation of MEA. Energy Proc. 2011, 4, 1566−1573. (9) Goldman, M. J.; Fine, N. A.; Rochelle, G. T. Kinetics of Nnitrosopiperazine formation from nitrite and piperazine in CO2 capture. Environ. Sci. Technol. 2013, 47, 3528−3534. (10) Douglass, M.; Kabacoff, B. The chemistry of nitrosamine formation, inhibition and destruction. J. Soc. Cosmet. Chem. 1978, 606, 581−606. (11) Challis, B.; Kyrtopoulos, S. The chemistry of nitrosocompounds. Part 11. Nitrosation of amines by the two-phase interaction of amines in solution with gaseous oxides of nitrogen. J. Chem. Soc. Perkin. 1979, 203, 299−304. (12) Cachaza, J. M.; Casado, J.; Castro, A. Compostela, E. De. Kinetic Studies on the Formation of Nitrosamines I: Formation of Dimethylnitrosamine in Aqueous Solution of Perchloric Acid. Z. flir Krebsforsch. Kiinische Onkol. 1978, 91, 279−290. (13) Challis, B.; Outram, J. The chemistry of nitroso-compounds. Part 15. Formation of N-nitrosamines in solution from gaseous nitric oxide in the presence of iodine. J. Chem. Soc., Perkin Trans. 1 1979, 2768−2775. (14) Challis, B. C.; Kyrtopoulos, S. A. Nitrosation under alkaline conditions. J. Chem. Soc. Chem. Commun. 1976, 877. (15) Lv, C.-L.; Liu, Y. D.; Zhong, R.-G. Theoretical investigation of N-nitrosodimethylamine formation from dimethylamine nitrosation catalyzed by carbonyl compounds. J. Phys. Chem. A 2009, 113, 713− 718. (16) Sun, Z.; Liu, Y. D.; Zhong, R. G. Carbon dioxide in the nitrosation of amine: catalyst or inhibitor? J. Phys. Chem. A 2011, 115, 7753−7764. (17) Closmann, F. B. Oxidation and thermal degradation of methyldiethanolamine/piperazine in CO2 capture. Ph.D. Dissertation, University of Texas at Austin, 2011. (18) Freeman, S. A. Thermal degradation and oxidation of aqueous piperazine for carbon dioxide capture. Ph.D. Dissertation, University of Texas at Austin, 2011. (19) Nielsen, P. T.; Li, L.; Rochelle, G. T. Piperazine degradation in pilot plants. Energy Proc. 2013, 37, 1912−1923. (20) Kim, I.; Jens, C. M.; Grimstvedt, A.; Svendsen, H. F. Thermodynamics of protonation of amines in aqueous solutions at elevated temperatures. J. Chem. Thermodyn. 2011, 43, 1754−1762. (21) Hamborg, E. S.; Versteeg, G. F. Dissociation Constants and Thermodynamic Properties of Amines and Alkanolamines from (293 to 353) K. J. Chem. Eng. Data 2009, 54, 1318−1328. (22) Fine, N. A.; Goldman, M. J.; Nielsen, P. T.; Rochelle, G. T. Managing n-nitrosopiperazine and dinitrosopiperazine. Energy Proc. 2013, 37, 273−284. (23) Walters, C. L.; J, H. R.; Smith, P. Analysis of roral N-Nitroso compounds as a group by denitrosation to nitric oxide, with detection using a chemiluminescence analyser. IARC Sci. 1983, 45, 295−308. (24) Williams, D. Quantitative aspects of nitrosamine denitrosation. In Nitrosamines and Related N-Nitroso Compounds; ACS Symposium Series; American Chemical Society: Washington, D.C., 1994; pp 66− 73. (25) Ding, Y.; Lee, M.; Eatough, D. The determination of total nitrite and N-nitroso compounds in atmospheric samples. Int. J. Environ. Anal. Chem. 1998, 69, 243−255. (26) Fine, N. A.; Nielsen, P. T.; Rochelle, G. T. Decomposition of nitrosamines in CO2 capture by aqueous piperazine or monoethanolamine. Environ. Sci. Technol. 2014, 48, 5996−6002. (27) Mitch, W. Critical Literature Review of Nitrosation/Nitration Pathways http://www.gassnova.no/frontend/files/CONTENT/ Rapporter/NitrosamineandNitramineformationchemistry_YALE.pdf (accessed on February 7, 2012). (28) da Silva, E. F.; Grimstvedt, A.; Vevelstad, S. J.; Einbu, A.; Vernstad, K.; Svendsen, H. F.; Zahlsen, K. Understanding 2 Ethanolamine Degradation in Postcombustion CO2 Capture. Ind. Eng. Chem. Res. 2012, 51, 13329−13338.

have the highest nitrosamine concentration. Almost 100% of absorbed NOx will form nitrosamines due to slow nitrosation of the tertiary amine. Tertiary amines themselves are less thermally stable than PZ. Thus, desorber temperature must be lowered, and the nitrosamine does not readily decompose. These results show the relative hazard of nitrosamine formation for three common solvent types as well as the importance of limiting MEA degradation from a nitrosamine accumulation perspective. steady state NNO =

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G⎞ δ ⎛⎜ NO2Abs ⎟ ⎝ kDesτDes L⎠

(18)

ASSOCIATED CONTENT

S Supporting Information *

Chemical list, quantification of NHeGly and NHEEDA, and list of uncommon abbreviations and symbols. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare the following competing financial interest(s): One author of this publication consults for Southern Company and for Neumann Systems Group on the development of amine scrubbing technology. The terms of this arrangement have been reviewed and approved by the University of Texas at Austin in accordance with its policy on objectivity in research. The authors have financial interests in intellectual property owned by the University of Texas that includes ideas reported in this paper.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Texas Carbon Management Program in the preparation of this work.

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REFERENCES

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